| The unique high altitude,large temperature difference and strong freeze-thaw conditions of the Qinghai-Tibet Plateau have caused widespread problems such as slope gully and surface slump in the permafrost subgrade slope of the plateau.Different from the uneven deformation of the roadbed,roadbed cracks and other diseases,slope diseases will not affect the normal operation of the road in a short time,but with the annual cycle of freeze-thaw cycles and frequent freeze-thaw cycles under the alternating effects of positive and negative temperatures in the cold season.If the maintenance is not in place or encounters heavy rainfall and largescale snow melting,it will cause slope collapse,insufficient roadbed width and other diseases,affecting the safety of the road.In this paper,the formation mechanism of subgrade slope disease in plateau permafrost is studied by means of data collection,field investigation,laboratory test and numerical simulation.(1)Based on the Gongyu Expressway,G214,and S224 roads,typical forms of roadbed and pavement diseases in permafrost are summarized and summarized.The roadbed diseases mainly include uneven deformation and cracks in the roadbed;The main types of road surface diseases are transverse cracks,pushing bumps,etc;Slope diseases mainly include loose soil,gullies,and surface collapse.On this basis,the causes and influencing factors of slope diseases were mainly analyzed,and the development process of slope diseases in plateau permafrost roadbed was described through a generalized diagram.(2)The climatic conditions in the plateau area were simulated indoors,and the soil freezethaw test device was modified to develop a test plan to reveal the soil porosity and mechanical properties under freeze-thaw cycles.The deformation of frost heave and thaw settlement is used to characterize the loose deformation of soil.The soil samples after different freeze-thaw cycles were scanned by electron microscope,the changes of soil microstructure were analyzed,and the mechanism of soil freeze-thaw porosity was demonstrated by generalization diagram.The change of shear strength of soil after freezing and thawing was measured by small direct shear test.(3)The freeze-thaw monitoring deformation of specimens with different compactness showed that the specimens with different compactness showed shrinkage in the first 3 or 4cycles of freeze-thaw cycles,and the larger the compactness,the smaller the shrinkage.With the increase of the number of freeze-thaw cycles,the expansion deformation occurs in succession,and tends to be basically stable after 9 freeze-thaw cycles.Through scanning electron microscopy analysis,it was found that the soil particles became broken with the increase of freeze-thaw cycles,and the internal pores gradually developed.(4)Through the direct shear test analysis of the specimens with different freeze-thaw cycles,it is found that the shear strength of the soil increases slightly in the shrinkage stage,which is due to the compactness of the soil.In the expansion stage,the shear strength decreases greatly due to the loose soil.After the 9th freeze-thaw cycle,the shear strength tends to be stable,indicating that the soil structure is basically stable.It is found that freeze-thaw porosity has a great influence on soil cohesion and little influence on internal friction angle.(5)The coefficient partial differential equation module(PDE)and solid mechanics module in COMSOL finite element software are used to establish a mathematical model for the three-field coupling of water and heat.The loose deformation of the soil during the freeze-thaw cycle is simulated and compared with the indoor test. |
PDF Full Text Request